Many types of neutron detectors have been developed, but all have limitations. 10B, 3He, and 6Li have long been used in neutron detectors, but the supply of 3He is extremely limited. In gas-based neutron detectors containing 10BF3 or 3He, the 10B or 3He serve as a converter medium to produce ionizing radiation from an incident neutron flux. A variety of 3He, 10B neutron detectors are known as well as scintillation detectors containing 6Li, but all suffer from the relatively low total number of boron, helium, or lithium atoms per unit volume. That is why neutron gas detectors are pressurized. Boron-lined proportional detectors are also widely used for detecting thermal neutrons. None of these detectors could approach the unit-volume density of boron atoms in solid-state boron detectors. Natural boron contains 19.6% of the isotope 10B and 80.4% of the isotope 11B, but only 10B interacts with neutrons, especially thermal neutrons, enabling the detection of neutrons via the reaction 10B(n, α) 7Li, and it is the alpha particles, α, and the lithium ions which are produced by this reaction that enable this isotope to be useful for the detection of neutrons.
Because of its large neutron cross section, 10B has long been used in neutron counters, based on the reaction:10B(n,α)7Li Q=2.8 MeVIn 3He detectors the reaction is:3He(n,p)3H Q=764 KeV.
Because the cross section for the reaction for 3He is about 1.4 times greater than that for the 10B reaction, current 3He neutron detectors are more sensitive than current 10B neutron detectors. In both types of gas-based neutron detectors (containing BF3 or 3He), the 10B or 3He serve as a converter medium to produce ionizing radiation from an incident neutron flux. In both cases their sensitivity is limited by the density of 10B or 3He atoms that are contained in the detector. That is why neutron gas detectors are pressurized. Even so, the density of detection nuclides in gas detectors (or current solid state detectors) cannot approach the density of the detection nuclides that can be achieved in a bulk solid, crystalline red boron detector. In gas detectors, measurement of the ionizing radiation produced by the interaction of neutrons with either 3He or 10B provides the measure of the incident neutron flux. A variety of 3He or 10B containing neutron detectors are known, but all suffer from the relatively low total number of boron or helium atoms that can act as neutron-to-ionizing radiation converters. Additionally, the national supply of 3He is extremely limited but 10B constitutes 19.78% of natural boron.
10B doping of standard semiconductors such as silicon or germanium does not produce sensitive neutron detectors because of the very low level of 10B they contain. Semiconductor neutron detectors have been severely handicapped by their low density of the neutron absorbing species, and attempts have been made to overcome this deficiency by the use of coatings, but such coatings must be thin.
In a pure crystalline boron or amorphous solid state pure boron detector, the density of boron atoms in the detector would be more than 500 times higher than that obtainable with 10B gas detectors pressurized to ten atmospheres and millions of times higher than current semiconductor neutron detectors that use boron as a dopant material. The semiconducting properties of beta rhombohedral boron, amorphous boron, tetragonal boron, and orthorhombic boron are extremely poor because of the deep and numerous trapping centers that exist in these materials due to their extremely complicated atomic structures. Because of these poor semiconducting properties, millions of electron-hole pairs produced by neutron interaction with the boron isotope 10B cannot be collected or produce any signal. Neutron detectors using these forms of boron consequently do not have good sensitivity.
U.S. Pat. No. 8,445,859 B2 to Wang et al., entitled “Neutron Detectors Comprising Boron Powder,” which is incorporated herein by reference, discloses the use of 10boron powder, 10boron carbide powder or combinations thereof directly deposited on a first conductive substrate for the detection of neutrons. Powdered boron is amorphous with no crystalline structure but its 10B isotope atoms react with neutrons to produce alpha particles. These alpha particles produce electron—hole pairs but amorphous boron is not a good semiconductor. The charge carriers which the interaction of 10B with neutrons in boron powder produces cannot be collected effectively for the same reasons they can't be collected efficiently in beta rhombohedral, tetragonal, or orthorhombic solid boron because these forms of boron have a high density of trapping centers. These trapping centers lead to the recombination of hole electron pairs before they can be collected by the measuring circuit. Wang, et al. do not disclose the use of crystalline alpha rhombohedral boron as a neutron detector.
U.S. Pat. No. 6,727,504 B1 to F. Patrick Doty, which is incorporated herein by reference, discloses the use of boron nitride as a solid state neutron detector. Doty discloses that boron nitride has a much higher concentration of boron atoms per unit volume than do BF3 neutron detector tubes, but boron nitride is also a very poor semiconductor, hence a majority of the hole—electron pairs produced by neutrons interacting with 10B atoms become trapped and annihilated, vitiating the advantages offered by the fact that the concentration of boron atoms in boron nitride is much higher than that in BF3 tubes. Doty does not disclose the use of crystalline alpha rhombohedral boron as a neutron detector
U.S. Pat. No. 6,771,730 B1 to Dowben et al., which is incorporated herein by reference, discloses the use of boron-carbide as a solid state neutron detector and method for using the same. Boron nitride has a greater density of boron atoms per unit volume than boron trifluoride gas detectors but the semiconducting properties of boron carbide are poor. Dowben, et al do not disclose a neutron detector that uses alpha rhombohedral red boron.
Accordingly, it would be desirable to provide a neutron detector having enhanced unit-volume sensitivity using crystalline alpha rhombohedral boron.